It is customary to assume that plasma accelerators are apparatuses designed for ionization of a working substance accompanied by acceleration of an ionized gas (plasma) under the action of electromagnetic force and gas pressure force upon generation of an electric discharge.
Plasma acceleration occurs in plasma accelerators as a result of an electric breakdown in an electrode-to-electrode gap. In steady-state plasma accelerators, the electric discharge time is sufficiently long—the typical breakdown time t is at least 1 second. In pulsed plasma accelerators, the electric discharge is of shorter duration. The pulsed discharge time t is about 1-100 μs.
Pulsed plasma accelerators are currently employed as actuating systems in spacecraft control systems and as pulsed low-temperature plasma injectors.
It is common knowledge that in order to maintain a spacecraft in a desired orbital position during retarding in a relatively dense residual atmosphere of the outer space, it is advantageous to use small-sized propulsion units with low power consumption. Such requirements are satisfied with the use of propulsion units based on pulsed plasma accelerators. Most of such propulsion units use solid dielectric as a working substance releasing gaseous products as a result of ablation under the action of thermal and radiant energy of an electric discharge generated.
There is a great tendency nowadays to a wide employment in the outer space of low-orbiting (with orbit height Horb=400-1 000 km) light-weight and small-sized spacecrafts of relatively simplified construction and low cost, said spacecrafts having the typical weight in the range of from 50 kg to 500 kg. However, these light-weight and small-sized spacecrafts have substantially restricted power supplying capacities of electric propulsions providing high accuracy in keeping of orbital parameters of both individual spacecrafts and groups of such spacecrafts. For this purpose, highly efficient small-sized electric propulsions capable of correcting and stabilizing of spacecraft orbits at the minimal power consumption are demanded.
Steady-state plasma accelerators used as spacecraft controlling electric propulsions have a number of grave disadvantages including the complexity of plasma accelerator construction, the complexity of a manufacture process and operation of an accelerator, the increased manufacture and operating costs, as well as insufficient propulsive efficiency (plasma acceleration efficiency) and low performance reliability at the power consumption of less than 150 W.
An ablation pulsed plasma accelerator is the most promising propulsion for a spacecraft with regard to the simplicity of construction, reliability, low costs and proper functioning at the power consumption of from several watts to hundreds of watts. A pulsed plasma accelerator also provides for maximal accuracy of spacecraft control as compared to other kinds of propulsion units used as actuating systems. However, the efficiency of pulsed plasma accelerators of the prior art does not meet the operative requirements for handling the majority of spacecraft control problems.
A substantial increase in the operating efficiency of a pulsed plasma accelerator, primarily over the range of power consumption of from 20 W to 300 W, within which the basic problems for controlling of spacecraft orbital parameters are currently solved and will be solved in the near future, is of fundamental importance for widening the range of functioning of a spacecraft.
Presently the basic technical problems of a pulsed plasma accelerator are the excessive retardation of evaporation of a working substance with regard to a discharge current and, as a result, ineffective acceleration of a substantial part of plasma generated, which on the whole adversely affects the efficiency of accelerator (the efficiency of plasma acceleration).
It has already been pointed out in the very first studies on the investigation of plasma acceleration processes in a pulsed plasma accelerator (Artsymovitch L. A. et al, “Electrodynamic acceleration of plasma coagulates”. ZHETF, Moscow, 1957, vol 33, No. 1) that the plasma acceleration efficiency is dependent upon dimensionless parameter q:q=l2C2U20/2mL0,                where l [H/m] is the linear inductance of accelerator electrodes;        C [F] is the capacity of an external discharge circuit;        U0 [V] is the initial voltage of an external discharge circuit;        m [kg] is the weight of a plasma coagulate;        L0 [H] is the initial inductance of an external discharge circuit.        
The physical meaning of the parameter q lies in defining the ratio of a typical value of a magnetic pressure force to a typical value of an accelerated plasma coagulate inertia force. It has been established that an increase in the parameter q results in that a discharge approaches a nonperiodic shape with rising of plasma acceleration efficiency.
One of the known features of a pulsed plasma accelerator is that weight m of accelerated plasma is commonly proportional to the power Wo applied to the discharge:m≈kW0,                where W0=CU20/2;        k=10−8-10−9 kg/J is a proportionality factor.        
When the dependence for W0 is introduced in the previous ratio, the dependence for the parameter q assumes the form of:q=12C2U20/2k(CU20/2)L0=(12/k)(C/L0).
So, with the assigned configuration and sizes of an accelerating channel of a pulsed plasma accelerator, the efficiency of plasma acceleration is characterized by the ratio of C/L0.
The specific technical solutions aimed at increasing the efficiency of plasma acceleration by means of a pulsed plasma accelerator and associated with the realization of theoretical reasoning of q of about C/Lo are not yet developed.
As an example, it is renowned a pulsed plasma accelerator designed for use as an electric propulsion of a system for controlling the position of a geostationary earth orbit satellite of a global communication system (A. I. Rudikov, N. N. Antropov, G. A. Popov. “Pulsed Plasma Thruster of Erosion Type for a Geostationary Artificial Earth Satellite”, 44th Congress of the International Astronautical Federation, IAF-93-S.5.487, Graz, Austria: IAF, Oct. 16-22, 1993). Propulsive pulses generated by such a propulsion unit must neutralize the effects of outer factors upon a spacecraft in a geostationary orbit.
Each pulsed plasma accelerator incorporated in a propulsion unit of the prior art comprises electrodes (a cathode and an anode), one of the electrodes being made in the form of a copper rod and the other of the electrodes being made in the form of a plate, a solid dielectric working substance ablating under the action of an electric discharge, a system for supplying of a working substance into a rail-type discharge channel, and a discharge-initiating system. Power is supplied to accelerator electrodes via current supplies from an outer energy accumulator of 36 μF capacitance at the maximal voltage of about 3 kV.
Such an accelerator operates at a gas pressure less than 10-4 torr in an accelerating channel. The energy released with each pulse is about 160 J at the current pulse amplitude of 35 kA. The disadvantage of the given propulsion unit is low propulsive efficiency, which is less than 10%, owing to an oscillating nature of a discharge current variation during each pulse time.
In another pulsed plasma accelerator of the prior art (P. J. Turchi “Directions for Improving PPT Performance”, 25th International Electric Propulsion Conference, IEPC 97-038, USA, Cleveland, Ohio: IEPC, Aug. 24-28, 1997), pulsed oscillating discharges were generated in a discharge channel with power being supplied to electrodes from a high-current capacitive accumulator. The stored energy of the accumulator was 20 J at an initiating voltage of 2 kV, and the accumulator capacitance was 10 μF. The inductance of an external electric circuit was 400 nH. However, despite the attempts of increasing the pulse time and creating a quasi-continuous discharge current at each pulse, the total propulsive efficiency of the propulsive unit did not reach 10%. The obtained propulsive efficiency does not allow such plasma accelerators to be employed in commercial spacecrafts.
With regard to the paper discussed, it should be mentioned that a proper conclusion was drawn on the need for coordination of impedances for internal and external circuits in a pulsed plasma accelerator. However, quite complicated and low-effective solutions are offered for handling the given problem, said solutions including the incorporation of additional components in the electric circuit. The mentioned components, such as capacitors, inductance coils and commutators, allow the internal and external circuits to be coordinated and a quasi-nonperiodic discharge in a pulsed plasma accelerator to be obtained, though the positive effect is substantially reduced by power losses associated with such components.
Apart from the above-mentioned, there exists other viewpoint concerning with an increase in the efficiency of a pulsed plasma accelerator. As an example, a pulsed plasma accelerator (propulsion) is known which comprises an accelerating channel defined by two electrodes, an insulator adapted for separating these electrodes and serving as a working substance, a discharge-initiating system, and an energy accumulator based on high-current capacitors and connected to the electrodes by means of a current supply. The given propulsion uses teflon as a working substance (Gregory G. Spanjers et al. “Investigation of Propellant Inefficiencies in a Pulsed Plasma Thruster”, AiAA-96-2723, 32nd JPC, Lake Buena Vista, Fla., USA: AIAA/ASME/SAE/ASEE, Jul. 1-3, 1996). During operation of the propulsion, the influence of electric discharge energy upon the efficiency of use of a working substance was investigated. However, despite the resultant increase in a propulsive pulse and propulsion value, the total propulsive efficiency of the plasma accelerator at the discharge energy of about 40 J varied from 7% to 8%. The relatively low propulsive efficiency was due to the oscillating nature of the discharge current variation during each pulse.
The conclusion was drawn in the discussed paper that in order to increase an efficiency of a pulsed plasma accelerator, the first one half-period time of a discharge current must be reduced and its amplitude must be increased. The above conclusion was supported by reliable experimental results, however it did not take into account nonlinearity of processes occurred in the input electric circuit of the pulsed plasma accelerator and caused by plasma.
In order to increase propulsive characteristics of pulsed plasma accelerators (propulsions), accelerators were designed for high level of electric discharge energy (W. J. Guman and D. J. Palumbo “Pulsed Plasma Propulsion System for North-South Stationkeeping”, AIAA-76-999, AIAA International Propulsion Conference, Key Biscayne, Fla., USA: AIAA, Nov. 14-17, 1976). A known pulsed accelerator (propulsion) includes two electrodes defining an accelerating channel, dielectric bars made from teflon and arranged between the electrodes, a ceramic end insulator, and a capacitive accumulator. The capacitance of the accumulator was rated for the generation of an electric discharge of about 750 J in the accelerating channel.
The discharge generated in the discharge channel of the present plasma accelerator is of oscillating type. The maximal total propulsive efficiency of the propulsion at the discharge voltage of 2.5 kW was 25.6%. However, with the indicated level of discharge energy the propulsive efficiency of the plasma propulsion may not be accepted sufficient since the efficiency of competitive plasma (magnetic plasma) propulsions, such as steady-state plasma propulsions, is up to 45% at this energy level.
It is known a pulsed plasma accelerator (propulsion) comprising two flat copper electrodes, two dielectric bars manufactured from an ablating material and arranged between the said electrodes, a discharge-initiating device, and an energy accumulator (N. Antropov et al. “Parameters of Plasmoids injected by PPT”, AIAA 97-2921, 33rd AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Seattle, Wash., USA: AIAA/ASME/SAE/ASEE, Jul. 6-9, 1997). An accelerating channel of the plasma accelerator is defined by surfaces of the electrodes and side surfaces of the dielectric bars. The energy accumulator includes five high-current capacitors with the total stored energy of 80-100 J. The operating voltage of the capacitor battery is 2.5-2.8 kV. The inductance of an external electric circuit connected to the electrodes of accelerator was 20 nH. The efficiency of plasma accelerator did not exceed 13% with the energy of electric discharge of 100 J.
The closest analog of the claimed invention is an erosion (ablation) plasma propulsion (accelerator) disclosed in Pat. No RU 2143586 C1 (IPC-6 F03H1/00, H05H1/54, published 27 Dec. 1999). The known analog includes electrodes (a cathode and an anode), which are connected via an ohmic and inductance load to a capacitor (energy accumulator) plates, a ceramic end insulator, which separates the electrodes from one another, and dielectric bars made from ablating material and arranged between the electrodes. The energy accumulator is connected to the electrodes through thin copper busbars (current supplies). The discharge channel walls are defined by the surfaces of electrodes and dielectric bars. The electrodes of the known plasma accelerator are made in the form of plates. A discharge-initiating device (igniter) is located in a slot formed in the end insulator.
The dielectric bars used in the known plasma accelerator are movable toward a discharge channel midline by means of a special moving device (a spring pusher). The dielectric bars are caused to move until abutment against a stop made in the form of a step on the surface of electrode.
Plasma is accelerated in the discharge channel of the plasma accelerator in the following manner. A narrow high-voltage pulse is supplied from a discharge-initiating unit to the electrodes of the discharge-initiating device. A surface breakdown results in generating of a plasma coagulate, which causes short-circuiting of the electrodes in the slot of the end insulator where an electric arc discharge is created. During breakdown, electrodes are at a “waiting” potential. A working substance is evaporated from the surfaces of the dielectric bars by radiant discharge energy, ionized and accelerated by electromagnetic force and gas dynamic pressure.
During operation of a plasma accelerator—analog, cord-shaped stable plasma is generated at the leading end of the accelerating channel to inhibit deposition of a carbon film in this part of the channel and, accordingly, eliminate non-uniform consumption of the working surface of dielectric bars. This phenomenon enhances the stable propulsive characteristics of the accelerator due to the uniform evaporation of the working substance.
The electric discharge in the accelerating channel of the plasma accelerator is of oscillating nature, with the number of one-half periods of pulse discharge current variations being three. As a result, the maximal propulsive efficiency of the plasma accelerator does not exceed 14%.
With the known ablation pulsed plasma accelerator, one of the greatest problems immediately affecting the efficiency characteristics of the accelerator are the working substance losses occurring in the accelerating channel during the plasma acceleration process.
The reason for the working substance losses has to do with space and time discrepancies of the two processes occurring in the accelerating channel of the pulsed plasma accelerator:                a relatively fast process (tpr of about 1.5-3 μs) of formation and acceleration of a discharge current region (a current arc);        a relatively slow process of heating the working surfaces of the working substance bars, ionization of the working substance, generation of a plasma flow and acceleration thereof (tpr is about 7-12 μs).        
The total oscillatory electric discharge time of the known pulsed plasma accelerator-analog is 8-15 μs depending on the sizes of the accelerating channel and the features of the discharge circuit. However, as it had been established, an effective electromagnetic process for plasma acceleration occurred only during the first discharge of the accumulator (the first one-half period of the discharge current), with the time of the said discharge making from 1.5 to 3.0 μs depending on the energy and sizes of the accelerator. Furthermore, in the course of the discharge process, only the ablation (evaporation) of the working substance and thermal (gas dynamic) plasma acceleration had occurred.